You can post it to:
U.S. Department of Homeland Security
U.S. Citizenship and Immigration Services
Nebraska Service Center
P.O. Box 87765
Lincoln, NE 68501-7765
Just as a general advice to all, this is a very commen question. Please use the search option before posting such questions. There will be dozens of threads regarding this. Also for address, fees and other FAQ's the best source is the USCIS site. In the time it took you to type the question, you could have gone to uscis.gov and got the answer yourself. Look in the forms and fees section, and select the appropriate form.